RR Lyrae Stars

The star RR Lyrae as it appears in the Deep Sky Survey

What is an RR Lyrae?

An RR Lyrae is a special type of star that changes in brightness with a regular period of a few days. They are called RR Lyraes because a type of variable star is named after the first star of that type discovered. The first known RR Lyrae variable was the star RR Lyrae.

How is an RR Lyrae different from our Sun?

RR Lyraes tend to be stars a little over half the mass of our Sun. Most RR Lyraes are probably much older than our Sun. RR Lyraes are much hotter than our Sun. They are in a stage of their life where they have expended the hydrogen in their core and are now burning helium into carbon through nuclear fusion processes. Our Sun burns hydrogen in its core into helium to produce energy. Also, unlike the Sun, RR Lyraes pulsate, growing larger and smaller, changing their brightness.

Why do RR Lyraes change in brightness?

The amount of light a star emits is dependent on two things: the surface area of the star and the temperature of the star.

If you had two stars of the same temperature but different sizes and put them at the same distance from you, the star that is larger would be brighter. If you had two stars that were the same size but one was hotter than the other, and you put them at the same distance from you, the hotter star would appear to be brighter.

RR Lyraes are pulsating, their surface moves radially in and out with a regular rhythm, a little like a balloon that is getting inflated and then deflated at regular intervals. From what we learned above, the changing size of the RR Lyrae should make it brighter when it is biggest and fainter when it is smallest, but we haven't accounted what is happening to the temperature of the RR Lyrae while it pulsates.

As the RR Lyrae shrinks, the surface heats up, like a piston compressing air into a small volume. Then as the RR Lyrae's surface expands it cools. We know from the speed at which RR Lyraes pulsate that their size cannot be changing enough to cause the change in brightness that we see. In order to do this the surface of the star would have to move in and out so fast that the star would blow it self apart. The temperature of the RR Lyrae's surface changes enough to account for the change in brightness though. So when the RR Lyrae is smallest in size and hottest, then it is brightest. When the RR Lyrae is largest in size and coolest, then it is at it's faintest.

What is "special" about RR Lyraes?

RR Lyraes are helpful to astronomers because if we know the period of time it takes for an RR Lyrae to go through its cycle of brightening and dimming, then we can figure the absolute luminosity of that star. The absolute luminosity tells us how bright a star would be if it was a certain distance away from us. If we measure how bright the star appears to us and compare that to its absolute luminosity, then we can determine the distance to the star. The distance to most stars cannot be determined with the accuracy we can determine the distance to RR Lyraes. This makes RR Lyraes special.

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